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#![no_std]
use core::borrow::Borrow;
use core::hash::Hash;
use core::num::NonZeroUsize;
use core::{cmp, mem};
use hashbrown::HashMap;
use polonius_the_crab::{polonius, polonius_return};
/// An LRU Cache
pub struct LruCache<K, V> {
l1_map: HashMap<K, V>,
l2_map: HashMap<K, V>,
cap: NonZeroUsize,
flips: usize,
}
impl<K: Hash + Eq, V> LruCache<K, V> {
/// Creates a new LRU Cache that holds `cap` items.
/// It can fetch upto the last `cap*2` items, but only
/// the last `cap` items is guaranteed to be in the cache.
///
/// When the cache is full (reached cap items), then a "flip" occurs internally,
/// where the full cache is backed up and an empty cache is brought in its place.
/// Then as cache misses occur, the cache gets populated internally from the backup
/// cache if the item is found there or a miss is reported to the user.
///
/// # Example
///
/// ```
/// use fliplru::LruCache;
/// use std::num::NonZeroUsize;
/// let mut cache: LruCache<isize, &str> = LruCache::new(NonZeroUsize::new(10).unwrap());
/// ```
pub fn new(cap: NonZeroUsize) -> LruCache<K, V> {
LruCache {
l1_map: HashMap::with_capacity(cap.into()),
l2_map: HashMap::with_capacity(cap.into()),
cap,
flips: 0,
}
}
/// Returns a reference to the value of the key in the cache or `None` if it is not
/// present in the cache.
///
/// # Example
///
/// ```
/// use fliplru::LruCache;
/// use std::num::NonZeroUsize;
/// let mut cache = LruCache::new(NonZeroUsize::new(2).unwrap());
///
/// cache.put(1, "a");
/// cache.put(2, "b");
/// cache.put(2, "c");
/// cache.put(3, "d");
///
/// assert_eq!(cache.get(&2), Some(&"c"));
/// assert_eq!(cache.get(&3), Some(&"d"));
/// ```
pub fn get<'a, Q>(&'a mut self, k: &Q) -> Option<&'a V>
where
K: Borrow<Q>,
Q: Hash + Eq + ?Sized,
{
let mut this = self;
polonius!(|this| -> Option<&'polonius V> {
if let Some(v) = this.l1_map.get(k) {
polonius_return!(Some(v));
}
});
match this.l2_map.remove_entry(k) {
Some((rk, rv)) => {
this.put(rk, rv);
this.l1_map.get(k)
}
None => None,
}
}
/// Returns a mutable reference to the value of the key in the cache or `None` if it
/// is not present in the cache. Moves the key to the l1_map if it exists in the l2_map.
///
/// # Example
///
/// ```
/// use fliplru::LruCache;
/// use std::num::NonZeroUsize;
/// let mut cache = LruCache::new(NonZeroUsize::new(2).unwrap());
///
/// cache.put("apple", 8);
/// cache.put("banana", 4);
/// cache.put("banana", 6);
/// cache.put("pear", 2);
///
/// assert_eq!(cache.get_mut(&"apple"), Some(&mut 8));
/// assert_eq!(cache.get_mut(&"banana"), Some(&mut 6));
/// assert_eq!(cache.get_mut(&"pear"), Some(&mut 2));
/// ```
pub fn get_mut<'a, Q>(&'a mut self, k: &Q) -> Option<&'a mut V>
where
K: Borrow<Q>,
Q: Hash + Eq + ?Sized,
{
let mut this = self;
polonius!(|this| -> Option<&'polonius mut V> {
if let Some(v) = this.l1_map.get_mut(k) {
polonius_return!(Some(v));
}
});
match this.l2_map.remove_entry(k) {
Some((rk, rv)) => {
this.put(rk, rv);
this.l1_map.get_mut(k)
}
None => None,
}
}
/// Puts a key-value pair into cache. If the key already exists in the cache, then it updates
/// the key's value and returns the old value. Otherwise, `None` is returned.
///
/// # Example
///
/// ```
/// use fliplru::LruCache;
/// use std::num::NonZeroUsize;
/// let mut cache = LruCache::new(NonZeroUsize::new(2).unwrap());
///
/// assert_eq!(None, cache.put(1, "a"));
/// assert_eq!(None, cache.put(2, "b"));
/// assert_eq!(Some("b"), cache.put(2, "beta"));
///
/// assert_eq!(cache.get(&1), Some(&"a"));
/// assert_eq!(cache.get(&2), Some(&"beta"));
/// ```
pub fn put(&mut self, k: K, v: V) -> Option<V> {
if self.l1_map.len() == self.cap.into() {
mem::swap(&mut self.l2_map, &mut self.l1_map);
let _ = mem::replace(&mut self.l1_map, HashMap::with_capacity(self.cap.into()));
self.flips += 1;
}
// invalidate any existing entry in L2 cache
let ov = self.l2_map.remove(&k);
match self.l1_map.insert(k, v) {
Some(l1_v) => Some(l1_v),
None => ov,
}
}
/// Returns the maximum number of key-value pairs the cache can hold.
///
/// # Example
///
/// ```
/// use fliplru::LruCache;
/// use std::num::NonZeroUsize;
/// let mut cache: LruCache<isize, &str> = LruCache::new(NonZeroUsize::new(2).unwrap());
/// assert_eq!(cache.cap().get(), 2);
/// ```
pub fn cap(&self) -> NonZeroUsize {
self.cap
}
/// Returns the number of key-value pairs that are currently in the the cache.
///
/// # Example
///
/// ```
/// use fliplru::LruCache;
/// use std::num::NonZeroUsize;
/// let mut cache = LruCache::new(NonZeroUsize::new(2).unwrap());
/// assert_eq!(cache.len(), 0);
///
/// cache.put(1, "a");
/// assert_eq!(cache.len(), 1);
///
/// cache.put(2, "b");
/// assert_eq!(cache.len(), 2);
///
/// cache.put(3, "c");
/// assert_eq!(cache.len(), 2);
/// ```
pub fn len(&self) -> usize {
cmp::min(self.l1_map.len() + self.l2_map.len(), self.cap().into())
}
/// Returns a bool indicating whether the cache is empty or not.
///
/// # Example
///
/// ```
/// use fliplru::LruCache;
/// use std::num::NonZeroUsize;
/// let mut cache = LruCache::new(NonZeroUsize::new(2).unwrap());
/// assert!(cache.is_empty());
///
/// cache.put(1, "a");
/// assert!(!cache.is_empty());
/// ```
pub fn is_empty(&self) -> bool {
self.l1_map.len() == 0 && self.l2_map.len() == 0
}
/// Returns metric on the number of times the cache became full.
///
/// # Example
///
/// ```
/// use fliplru::LruCache;
/// use std::num::NonZeroUsize;
/// let mut cache = LruCache::new(NonZeroUsize::new(2).unwrap());
///
/// for i in 0..5 {
/// cache.put(i, i);
/// }
/// for i in 0..20 {
/// cache.get(&(i % 5));
/// }
/// assert_eq!(cache.get_flips(), 8);
/// ```
pub fn get_flips(&self) -> usize {
self.flips
}
/// Reset the flip metric.
///
/// # Example
///
/// ```
/// use fliplru::LruCache;
/// use std::num::NonZeroUsize;
/// let mut cache = LruCache::new(NonZeroUsize::new(2).unwrap());
///
/// for i in 0..5 {
/// cache.put(i, i);
/// }
/// for i in 0..20 {
/// cache.get(&(i % 5));
/// }
/// assert_eq!(cache.get_flips(), 8);
/// cache.reset();
/// assert_eq!(cache.get_flips(), 0);
/// ```
pub fn reset(&mut self) {
self.flips = 0;
}
}
#[cfg(test)]
mod tests {
use super::LruCache;
use core::{fmt::Debug, num::NonZeroUsize};
fn assert_opt_eq<V: PartialEq + Debug>(opt: Option<&V>, v: V) {
assert!(opt.is_some());
assert_eq!(opt.unwrap(), &v);
}
#[test]
fn test_put_and_get() {
let mut cache = LruCache::new(NonZeroUsize::new(2).unwrap());
assert!(cache.is_empty());
assert_eq!(cache.get_flips(), 0);
assert_eq!(cache.put("apple", "red"), None);
assert_eq!(cache.put("banana", "yellow"), None);
assert_eq!(cache.cap().get(), 2);
assert_eq!(cache.len(), 2);
assert_eq!(cache.get_flips(), 0);
assert!(!cache.is_empty());
assert_opt_eq(cache.get(&"apple"), "red");
assert_opt_eq(cache.get(&"banana"), "yellow");
}
#[test]
fn test_put_update() {
let mut cache = LruCache::new(NonZeroUsize::new(2).unwrap());
assert_eq!(cache.put("apple", "red"), None);
assert_eq!(cache.put("apple", "green"), Some("red"));
assert_eq!(cache.len(), 1);
assert_opt_eq(cache.get(&"apple"), "green");
}
#[test]
fn test_l2() {
let mut cache = LruCache::new(NonZeroUsize::new(2).unwrap());
assert_eq!(cache.get_flips(), 0);
assert_eq!(cache.put("apple", "red"), None);
assert_eq!(cache.put("banana", "yellow"), None);
assert_eq!(cache.put("pear", "green"), None);
assert_eq!(cache.get_flips(), 1);
// This is retrieved from the overflow (L2 cache)
assert_opt_eq(cache.get(&"apple"), "red");
assert_opt_eq(cache.get(&"banana"), "yellow");
assert_opt_eq(cache.get(&"pear"), "green");
assert_eq!(cache.get_flips(), 2);
// apple is no longer in both the caches
assert_eq!(cache.put("apple", "green"), None);
assert_eq!(cache.put("tomato", "red"), None);
assert_eq!(cache.get_flips(), 3);
assert_opt_eq(cache.get(&"pear"), "green");
assert_opt_eq(cache.get(&"apple"), "green");
assert_opt_eq(cache.get(&"tomato"), "red");
assert_eq!(cache.get_flips(), 5);
}
#[test]
fn test_max_cache_len() {
let mut cache = LruCache::new(NonZeroUsize::new(2).unwrap());
assert_eq!(cache.put("apple", "red"), None);
assert_eq!(cache.put("banana", "yellow"), None);
assert_eq!(cache.put("pear", "green"), None);
assert_eq!(cache.put("tomato", "red"), None);
assert_eq!(cache.get_flips(), 1);
// Could retrieve `cap*2` oldest item, i.e., the 4th oldest item.
assert_opt_eq(cache.get(&"apple"), "red");
assert_eq!(cache.get_flips(), 2);
// Could not retrieve `cap+1` oldest item, i.e., the 3rd oldest item, showing that only the
// first `cap` items is guaranteed to be in the cache.
assert_eq!(cache.get(&"banana"), None);
assert_eq!(cache.get_flips(), 2);
cache.reset();
assert_eq!(cache.get_flips(), 0);
}
#[test]
fn test_cache_under_capacity() {
let mut cache = LruCache::new(NonZeroUsize::new(2).unwrap());
for i in 0..5 {
cache.put(i, i);
}
for i in 0..20 {
cache.get(&(i % 5));
}
assert_eq!(cache.get_flips(), 8);
}
#[test]
fn test_cache_over_capacity() {
let mut cache = LruCache::new(NonZeroUsize::new(5).unwrap());
for i in 0..5 {
cache.put(i, i);
}
for i in 0..20 {
cache.get(&(i % 5));
}
assert_eq!(cache.get_flips(), 0);
}
}